A Novel All-Solid-State Laser Source for Lithium Atoms and Three-Body Recombination in the Unitary Bose Gas Ulrich Eismann
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A novel all-solid-state laser source for lithium atoms and three-body recombination in the unitary Bose gas Ulrich Eismann To cite this version: Ulrich Eismann. A novel all-solid-state laser source for lithium atoms and three-body recombination in the unitary Bose gas. Quantum Gases [cond-mat.quant-gas]. Université Pierre et Marie Curie - Paris VI, 2012. English. tel-00702865 HAL Id: tel-00702865 https://tel.archives-ouvertes.fr/tel-00702865 Submitted on 31 May 2012 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. DÉPARTEMENT DE PHYSIQUE DE L’ÉCOLE NORMALE SUPÉRIEURE LABORATOIRE KASTLER-BROSSEL THÈSE DE DOCTORAT DE L’UNIVERSITÉ PARIS VI Spécialité : Physique Quantique présentée par Ulrich Eismann Pour obtenir le grade de Docteur de l’Université Pierre et Marie Curie (Paris VI) A novel all-solid-state laser source for lithium atoms and three-body recombination in the unitary Bose gas Soutenue le 3 Avril 2012 devant le jury composé de : Isabelle Bouchoule Rapporteuse Frédéric Chevy Directeur de Thèse Francesca Ferlaino Examinatrice AndersKastberg Rapporteur Arnaud Landragin Examinateur Christophe Salomon Membre invité Jacques Vigué Examinateur Contents General introduction 11 I High power 671-nm laser system 13 1 Introduction to Part I 15 1.1 Applications of 671-nm light sources ................... 16 1.2 Currently available 671-nm laser sources ................. 20 1.3 Alternative sources at 1342 nm ....................... 22 1.4 General design of the laser source and outline of Part I ......... 26 2 Diode-pumped all-solid-state Nd:YVO4 laser source 29 2.1 The Nd:YVO4 gain medium ........................ 31 2.1.1 Crystal strucure ........................... 31 2.1.2 Absorption ............................. 32 2.1.3 Emission ............................... 33 2.2 Laser theory ................................. 35 2.2.1 Hermite-Gaussian beams ...................... 35 2.2.2 Laser output power ......................... 38 2.2.3 Thermal effects in solid-state lasers ................ 39 2.2.4 Laser cavity design ......................... 41 2.3 Single-frequency operation and tunability ................. 45 2.3.1 Faraday rotation and unidirectional operation .......... 45 2.3.2 Etalons as frequency-dependent filters .............. 46 2.3.3 Etalon walk-off loss ......................... 47 2.3.4 Etalon choice and operation .................... 48 2.3.5 Etalon temperature tuning ..................... 50 2.3.6 Output spectrum .......................... 51 2.4 Output power ................................ 52 2.5 Spatial mode ................................ 55 2.6 Summary .................................. 57 3 Second harmonic generation 59 3.1 Theory of frequency doubling ....................... 60 3.1.1 Nonlinear conversion ........................ 60 3.1.2 Quasi-phase matching ....................... 61 3.1.3 Thermal effects and related dynamics ............... 62 i ii CONTENTS 3.2 Choice of the nonlinear medium ...................... 63 3.3 Single-pass measurements ......................... 65 3.4 Doubling cavity design ........................... 68 3.4.1 Mode matching and intra-cavity loss ............... 69 3.4.2 Impedance matching ........................ 71 3.5 Second harmonic output power and limitations .............. 72 3.5.1 Cavity characterization ....................... 72 3.5.2 Measurements of thermal effect and further optimization . 75 3.6 Summary .................................. 76 4 Frequency stabilization, characterization and implementation 77 4.1 Lock system ................................. 78 4.1.1 Cavity length actuation ...................... 78 4.1.2 Doubling cavity lock ........................ 79 4.1.3 Saturated absorption spectroscopy and laser lock ........ 80 4.1.4 Alternative locking schemes .................... 81 4.2 Characterization .............................. 82 4.2.1 Relative intensity noise ....................... 82 4.2.2 Linewidth .............................. 83 4.2.3 Spatial mode quality ........................ 84 4.2.4 Long-term stability and every-day operation ........... 84 4.3 Implementation in the current lithium experiment ............ 85 4.3.1 EOM sideband creation ...................... 86 4.3.2 Additional frequencies and light delivery ............. 88 4.3.3 Operation of Zeeman slower and MOT .............. 90 4.4 Summary .................................. 92 5 Conclusion to Part I 93 II Three-body loss in the unitary Bose gas 97 6 Introduction to Part II 99 7 Experimental setup 103 7.1 The 7Li atom ................................ 104 7.1.1 Energy level scheme ........................ 104 7.1.2 Hyperfine-ground-state energies .................. 105 7.1.3 The 737.8-G magnetic Feshbach resonance ............ 106 7.2 Experimental procedure .......................... 107 7.2.1 Zeeman slower, MOT and CMOT ................. 107 7.2.2 Quadrupolar magnetic trapping and transport .......... 109 7.2.3 Ioffe-Pritchard trap ......................... 109 7.3 Hybrid dipolar-magnetic trap ....................... 110 7.3.1 Trap frequency calibration 1: axial direction ........... 111 7.3.2 Trap frequency calibration 2: radial direction .......... 112 7.4 Imaging ................................... 113 CONTENTS iii 7.5 Creating strongly-interacting Bose gases ................. 114 7.5.1 Magnetic field sweeps ........................ 115 7.5.2 RF transfers ............................. 115 7.5.3 Magnetic field stability ....................... 116 7.6 Summary .................................. 118 8 The unitary Bose gas: Results and discussion 119 8.1 Three-body loss in the unitary Bose gas ................. 120 8.1.1 Experimental procedure ...................... 120 8.1.2 Decay rate equations ........................ 121 8.1.3 Suppression of N = 3-order loss .................. 122 8.1.4 Temperature-dependent results .................. 124 8.1.5 A criterion of collisional quasi-equilibrium ............ 127 8.2 Low-fugacity unitary Bose gas equation of state ............. 127 8.3 Summary .................................. 130 9 Conclusion to Part II 133 General conclusion 137 A A glance on theory 139 A.1 The N-body scattering problem ...................... 139 A.1.1 Two-body elastic scattering .................... 142 A.1.2 Unitarity-limited three-body loss ................. 143 A.2 Equation of state of the finite-temperature unitary Bose gas . 144 B Publications 147 Abstract In this thesis we present novel techniques for the study of ultracold gases of lithium atoms. In the first part of this thesis, we present the development of a narrow-linewidth laser source emitting 840 mW of output power in the vicinity of the lithium D-line resonances at 671 nm. The source is based on a diode-end-pumped unidirectional ring laser operating on the 1342-nm transition in Nd:YVO4, capable of producing 1.3 W of single-mode light delivered in a diffraction-limited beam. The output beam is subsequently frequency-doubled using periodically-poled potassium titanyl phosphate (ppKTP) in an external buildup cavity. We obtain doubling efficiencies of up to 86%. Tunability of the output frequency over more than 400 GHz and frequency-locking of the cavity ensemble with respect to the lithium D-line transitions are accomplished. +400 We measure the linewidth to be 200 200 kHz. − In the second part of this thesis, we employ the source in an experimental setup to produce to cool and trap lithium atoms. We realize samples of finite-temperature unitary Bose gases around the center of a Fano-Feshbach resonance, where interactions between the atoms are maximized. We present temperature-dependent measurements of the unitarity-limited three-body loss rate. The measured losses attain the limiting value imposed by quantum mechanics without adjustable parameters. This measure- ment allows for the introduction of a criterion for quasi-equilibrium. In this regime, by using technique based on in-situ imaging developed in our group, we provide a first measurement of the equation of state of the unitary Bose gas at low fugacities. Résumé Dans cette thèse, nous présentons des nouvelles techniques et leur application dans l’étude des gaz d’atomes de lithium ultrafroids. Dans la première partie de cette thèse, nous présentons le développement d’une nouvelle source laser de faible largeur spectrale, capable d’émettre 840 mW de puis- sance dans la gamme des longeurs d’ondes des raies D du lithium atomique à 671 nm. La source est basée sur un laser en anneau pompé par diode, fonctionnant sur la tran- sition à 1342 nm dans le Nd:YVO4, capable de produire 1.3 W de lumière monomode dans un faisceau limité par la diffraction. Le faisceau de sortie est ensuite doublé en fréquence dans un cristal de phosphate de potassium titanyl (ppKTP) périodiquement polarisé dans une cavité externe. Nous obtenons un rendement du doublage de 86%. Une accordabilité de la fréquence de sortie sur plus de 400 GHz et le verrouillage de l’ensemble des cavités par rapport aux raies D du lithium sont accomplis. Nous avons +400 mesuré la largeur de raie d’émission à 200 200 kHz. − Dans la deuxième